It well known to produce endless lengths of welded steel tubing from strip stock and to continuously galvanize that tubing by providing a zinc coating on the exterior surface as taught, for example, in U.S. Pat. Nos. 3,122,114 and 3,230,615 which are owned by the assignee of this patent application. It is likewise known to continuously apply polymeric coatings to the exterior of such continuously formed tubing, employing various thermoplastic and thermosetting resins, as for example taught in U.S. Pat. Nos. 3,559,280; 3,616,983; 3,667,095; 3,965,551 and 4,621,399.
A preferred method of providing a plastic coating on the exterior of the tubing is by passing the formed tubing through a preheating station and then through a powder coating station having a chamber including a particulate fluidized bed of thermoplastic particles or a spray chamber. The coating station is part of an in-line, continuous, roll-forming, tube mill production line including upstream stations where the steel strip is formed into a tubular configuration and the edges thereof welding together by, for example, an induction welder.
It is known to use nylons 11 and 12 for the plastic coating to provide decoration and for limited surface protection of the steel tubing. It is also known to use these nylons for applications where greater mechanical strength of the plastic coating is required, as where the coating is stretched due to bending of the tubing or where abrasion resistance and increased toughness is needed. When the nylon coating is intended to supply primarily mechanical strength as opposed to only decoration and when the nylon is applied to tubing moving at high speed and cured over time, the nylon coating should be applied and subjected to a temperature greater than its melting point for sufficient time for the nylon to flow and strongly bond to the tubing surface, thereby forming a matrix about the encircled tubing.
Whenever a nylon coating undergoes melt-flow, a high gloss surface finish is developed. For example, a coating formed of nylon 11 may have a specular gloss (measured as a percent of the incident light at a 60 degree angle per ASTM Method D523-67) of 70% reflectance. However, high gloss surface finishes are sometimes undesirable. Addition of pigments and fillers to the coating to obtain a matte surface finish reduces its elongation characteristics and limits fabrication of coated tubing.